> I wish that I had known about rackunit earlier. What other
> 'adult' features of Racket should people would you recommend to
> readers of HtDP?
When you think of objects in the Java or C++ sense, there are two
concepts there that are mixed together: interfaces and inheritance.
Racket's structure properties gets us interfaces, and they are
definitely not taught in an intro class. If we want to have a general
interface to poke a value, we can do so:
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
#lang racket
(define-values (prop:poke pokeable? pokeable-procedure)
(make-struct-type-property 'poke))
(define (poke! thing)
(cond [(pokeable? thing)
((pokeable-procedure thing) thing)]
[else
(error 'poke! "Don't know how to poke ~e" thing)]))
(struct mascot ()
#:property prop:poke (lambda (self)
(printf "Ho ho!\n")))
(define pillsbury-dough-boy (mascot))
(poke! pillsbury-dough-boy)
(struct cartoon-character (name)
#:property prop:poke (lambda (self)
(printf "~a: that tickles!\n"
(cartoon-character-name self))))
(define homer (cartoon-character "Homer"))
(poke! homer)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
There are a few structure property types that are built-into Racket,
to customize the way that values print as strings, to make a structure
look like a function, etc.